Thermal Changes of Root Surface of Anterior Primary Teeth in Pulpectomy with Er:YAG Laser.

Objectives
Successful root treatment depends on elimination of microorganisms from the root canal. Considering incomplete removal of bacteria from the canal by usual methods, lasers have been suggested as a new modality. Despite their anti-bacterial properties, lasers can cause thermal changes. This study assessed the thermal changes of root surface in pulpectomy of primary teeth following the use of Er:YAG laser.


Materials and Methods
Sixty primary anterior teeth were collected and prepared by K-file up to number 50. Then, they were randomly divided into two groups and were irradiated with Er:YAG laser. The first group was irradiated with 1 W laser and the second group with 1.5 W laser. The laser irradiation time was two 10-second cycles with a 2-second interval in both groups. Thermal changes were measured by a thermometer in the apical and coronal areas per second. The results were analyzed by repeated measures ANOVA considering the laser power as between-subject variable.


Results
There was a temperature increase in the coronal and apical areas in use of 1 W power. There was a temperature rise in the coronal and apical areas in use of 1.5 W power. The temperature rise in the apical third was more than that in the coronal third; also, the average temperature rise was more in use of 1.5 W power than 1 W power.


Conclusions
As the average temperature increase was not more than 7°C in any group, this type of laser seems to be suitable for root treatment of primary anterior teeth.


INTRODUCTION
Early loss of primary teeth can cause malocclusion, unesthetic appearance, speech problems and temporary or permanent functional impairment. It is imperative to preserve pulp vitality as much as possible [1]. However, root canal therapy in primary teeth may be required in some cases such as irreversible pulpitis and pulp necrosis in order to prevent damage to permanent teeth [2]. In some occasions, pulpectomy alone is not sufficient [3], and cleaning, shaping and filling of the canal with absorbable pastes yields successful results [4][5][6].
Microorganisms are mainly responsible for root canal treatment failure [7,8]. Manual cleaning and shaping along with the use of chemical irrigating solutions may not be able to completely eliminate the microorganisms from the infected root canals [9,10]. According to Peters [11], nearly half of the canal walls remain unprepared when using the manual NiTi and stainless steel systems. Different methods such as sonic and ultrasonic equipment have been suggested to improve the root canal irrigation quality [12,13]. Laser is another modality suggested for activation of irrigating solutions in 2009 [14]. Laser has many advantages such as no mechanical contact and not creating smear layer, and can be used for decontamination of root canal and control of bleeding [15][16][17]. Birang et al, [18]  Eriksson et al. [30] concluded that temperature increase of 10ºC for one minute was high enough for alveolar bone necrosis. Generally, 7ºC increase in temperature is considered as the tolerance threshold for periodontal tissues [33]. Regarding the morphologic and structural differences between primary teeth and permanent teeth, they may also be different in terms of thermal changes [34,35]. Another disadvantage of laser in dentistry could be the possibility of apical perforation of curved canals because of straight line of laser beam. According to Li et al, [36] Er:YAG laser could ablate dentin and enamel; thus, if the pulse reaches the dentin surface in a wrong angle, it could cause apical perforation; this limits the laser application to straight canals only. The effect of thermal changes caused by laser during pulpectomy of primary teeth has not yet been studied; thus, this study was conducted aiming to assess the effect of thermal changes caused by laser during pulpectomy of primary anterior teeth.

Sample preparation:
This study was conducted on 115 hopeless, severely decayed, primary anterior teeth. The study protocol was approved in the ethical committee of our university (code:32085). Sixty teeth with root resorption less than one-fourth of the root length were selected. The crowns of the teeth were cut by a diamond fissure bur such that the remaining root length was 10 mm. Then, the roots were cleaned and shaped to 9 mm working length using a K-file (Kerr, Orange, CA, USA) up to number 50. During cleaning and shaping, the canals were rinsed with 5.25% sodium hypochlorite. After preparation, the teeth were autoclave-sterilized at 134ºC for 15 minutes [37]. The teeth were kept in distilled water at room temperature after extraction.

Laser irradiation and temperature measurement:
The teeth were randomly divided into two groups.  They were placed in a Teflon mold with a height of 5 cm and diameter of 10 cm. A digital thermometer (ST-8891E; Standard, Hong Kong, China) with an accuracy of 0.1ºC and range of 30 to 550ºC, which had data logger was used. One of the thermometer probes was in contact with the root surface in the coronal third and the other probe was in contact with the apical third to measure the temperature in these areas. The other probe of the device was placed at room temperature to compare the thermal changes in root surface with the room temperature changes. The samples were irradiated with Er:YAG laser (Fidelis, Fotona, Slovenia) at a wavelength of 2940 nm, fiber length of 20 mm and diameter of 300 µ in spiral motion. Laser irradiation was started from the apical third 1 mm away from the radiographic apex and 9 mm away from the preparation endpoint. In the first group, laser with 1 W power, 100 mJ energy, 10 Hz frequency and energy density of 70.77 J/cm2 in short pulse mode (250 ms) was irradiated for 20 seconds (two 10-second cycles with a 2-second time interval). In the second group, laser with 1.5 W power, 150 mJ energy, 10 Hz frequency and energy density of 106.15 J/cm 2 in short pulse mode (250 ms) was irradiated for 20 seconds (two 10-second cycles with a 2-second time interval). The temperature was recorded at each second, sent to the computer automatically and saved in Thermometer-E software. The results were analyzed by repeated measures ANOVA considering the laser power as between-subject variable using SPSS version 22 (SPSS Inc., IL, USA). Figures 1 and 2 show the recorded temperatures from the root surface during laser irradiation (for two 10-second rounds with a 2-second time interval) with 1 and 1.5 W powers. Based on the results of repeated measures ANOVA (quasi-likelihood information criteria = 84.4), irradiation zone, time and laser power had a significant effect on temperature changes ( Table 1). The temperature increased by longer laser irradiation as shown in Figures 1 and 2. Table 1 shows the average temperature increase caused by 1 and 1.5 W powers of laser for 22 seconds. Based on the test results, in both tested powers, there was a significant difference between the cervical and apical thirds (P<0.001); the temperature increase in the apical third was  Figure 3 shows the range of thermal changes during 22 seconds for the apical and coronal thirds caused by 1 and 1.5 W laser.

DISCUSSION
Success of root canal treatment is directly related to the removal of microorganisms from the root canal system [38]; but complete removal of microorganisms from the root canal system is not possible after cleaning, shaping and irrigation in all cases [39][40][41]. Recently, laser was introduced as a method of decontamination and removing the canal debris [9, 39-41]. Many different types of lasers are used in pediatric dentistry [20]. Besides the many advantages of laser, it may cause some undesirable effects [42]. One of these undesirable effects is the thermal damage following temperature increase [43]. Determining the time required for canal decontamination in primary teeth is also important since longer duration of laser irradiation would result in higher temperature rise. More research is required on this field. Another way to prevent temperature increase in the PDL is to use laser periodically and not continuously [45]. In this study, laser irradiation was done in two 10-second cycles with a time interval of two seconds, which did not cause average temperature increase over the threshold of tissues. It should be mentioned that the temperature did not decrease in the time interval; however, different results may be obtained in the human body because of the blood vessels in the PDL [32]. Further studies are required to assess the suitable duration of laser irradiation.

CONCLUSION
According to the results, it can be concluded that using 1 and 1.5 W powers for 20 seconds (two 10-second cycles) is safe in root canal therapy. The thermal change was 4.1±1.81ºC and 1.57±0.953ºC for 1 W power and 6.25±0.63ºC and 3.04±0.45ºC for 1.5 W power in the apical and coronal thirds, respectively.